1. Field of the Invention
The present invention relates to a lithium-ion battery, and more particularly, to a prismatic lithium secondary battery having a finishing tape and a lower part tape.
2. Description of the Related Art
As the use of portable electronic devices increases, the use of the batteries for the power source is increasing. A secondary battery is expensive compared with a disposable battery, but it is economical as the battery can be reused by charging it. Therefore a secondary battery is popularly used for portable electronic devices as a power supply.
The lithium secondary battery among secondary batteries has advantages that it has a high energy capacity per unit volume or unit mass, and has longer lifespan because it has no memory effect, and has a high terminal voltage. Therefore, the lithium secondary battery is used for high end electronic devices such as notebook computers, camcorders, cellular phones and the like which requires a high mobility.
The lithium ion battery uses LiCoO2 (lithium cobalt oxide), LiNiO2 (lithium nickel oxide), Li2MnO4 (lithium manganese oxide), or lithium salt of an oxide of a solid solution containing at least two of cobalt, nickel and manganese, which are able to intercalate and deintercalate lithium-ion, for a positive active material. Some kinds of carbon structures, which are able to intercalate and disintercalate lithium-ion, are used for a negative active material. Each of these two electrode active materials is formed into slurry having a binder, solvent, conductive agent and slurry, and is applied to a surface of a collector to form an electrode.
A separator is interposed between two electrodes, and an electrolyte is provided as a medium through which lithium ions flows between the two electrodes. As the lithium battery has higher standard oxidation electric potential, the lithium battery operates in higher voltage. Therefore, the lithium battery requires a solution in which lithium salt is melted in the non-aqueous organic solvent for electrolyte. Aqueous electrolyte, which is containing water and generates electrolysis at voltage above 1.43 V, cannot be used as electrolyte for the lithium battery.
Because the mobility of ions in a non aqueous electrolyte is very low compared with the mobility of ions in an aqueous electrolyte, it is not easy to make the lithium battery flow a large amount of current, even though the terminal voltage of the lithium ion battery is high. In order to solve this problem, a method increasing facing areas of two electrodes can be used. In this method, electrodes in lithium ion battery can be formed in several layers and electrode layers having opposite polarities can be alternately stacked. Alternatively, the electrodes are stacked and wound to make a jelly-roll type electrode assembly.
The lithium secondary battery is classified into a can type lithium ion secondary battery, in which a case storing an electrode assembly is a metal can in a predetermined shape, and a pouch type lithium ion secondary battery, in which the case is a pouch that is thin and flexible. The can type lithium ion secondary battery is classified into a cylinder type battery, and a prismatic type battery which is proper to reduce its volume and increases portability.
Demand for small size, light-weight and high capacity has continuously increased in lithium ion battery. To meet these requirements simultaneously, it is necessary to increase the amount of the active material to increase charge capacity in a limited volume of the battery, and to reduce the volume or amount of other materials such as a separator, a coupling material (binder) and the like, which compose an electrode assembly. It is also necessary to increase an area of a surface on which an active material contacts electrolyte, which will increase the amounts of the active material and electrolyte contacting each other.
When the electrode assembly is wrapped with a finishing tape and a lower part tape to secure the electrode assembly, the finishing tape and the lower part tape do not affect the electric charging capacity of the lithium ion battery, and therefore the increased volume of the finishing tape and the lower part tape relatively decreases the volumes or amounts of materials that affect the electric charging capacity of the lithium ion battery, which results in bringing down electric charging capacity per unit volume or unit mass. Additionally, the consumption of the tape increases the manufacturing cost of the battery.